1. Field of the Invention
The present invention relates to a PDS (Passive Double Star) structured optical subscriber system and in particular, to a PDS structured optical subscriber system in which the length of an entrance use slot is halved.
2. Description of the Prior Art
As shown in FIG. 4, in a PDS structured optical subscriber system, an optical output signal of optical burst signal transmitting portion 11 of center unit 1 is branched to N signals, wherein N is any integer larger than 0, by an optical branching/coupling unit (hereinafter referred to as star coupler 2). The branched optical signals are inputted to optical subscriber units 41 to 4n. Optical output signals of optical subscriber units 41 to 4n are coupled by star coupler 3. The coupled optical signal is inputted to optical burst signal receiving portion 12 of center unit 1.
FIGS. 5A and 5B show frame formats used in the PDS structured optical subscriber system. In FIGS. 5A and 5B, the length of each frame is fixed, in which each frame has slots corresponding to optical subscribers and an entrance use slot. As shown in FIG. 5A, in a downbound frame, an overhead is added. Each optical subscriber unit receives only data of a slot assigned thereto from among slot 1 to slot n of a downbound frame format. In addition, each optical subscriber unit transmits data to a slot assigned to itself among slot 1 to slot n of an upbound frame format. Although the upbound frame format is formed by star coupler 3, the delay time period of an optical signal from transmission at each optical subscriber unit 4 to arrival at star coupler 3 is different from one another since a distance between unit 4 and star coupler 3 is different from one another. Thus, each optical subscriber unit 4 needs to adjust the output timing of data to the relevant slot. When each optical subscriber unit 4 newly subscribes to the system, center unit 1 assigns the output timing to optical subscriber unit 4.
Center unit 1 places burst signal at the beginning of the entrance use slot of the downbound frame format. When optical subscriber unit 4m (1xe2x89xa6mxe2x89xa6n) that newly subscribes to the system receives the burst signal, it sends the burst signal back to center unit 1. When center unit 1 receives the burst signal from optical subscriber unit 4m, delay measuring portion 182 of center unit 1 measures the turnaround delay time period of the burst signal in the entrance use slot. Timing generating portion 182 of center unit 1 calculates the response timing for output data of optical subscriber unit 4m corresponding to the measured delay time period. The response timing data is placed in the entrance use slot of the downbound frame format. After receiving the response timing data, optical subscriber unit 4m transmits data to center unit 1 keeping the timing corresponding to the response timing data.
In the above-explained prior art, when an optical subscriber unit newly subscribes to the system, the downbound frame format and the upbound frame format have respective entrance use slots as shown in FIGS. 5A and 5B so as to prevent initial response burst signal of a new subscriber unit from colliding with service data of other optical subscriber units that have subscribed to the system. Since the delay time period of initial return optical burst signal of the optical subscriber unit 4m is not controlled by the center unit 1, the position of the initial return optical burst signal in the upbound frame format is indefinite. Thus, the entrance use slot needs to have a length corresponding to the delay time period spent in an maximum allowable installation distance between center unit 1 and each optical subscriber unit.
Consequently, when the maximum allowable installation distance becomes large, the length of the entrance use slot becomes long. In other words, the length of service data in the frame format becomes short. Assuming that the maximum allowable installation distance is 10 km, the length of the entrance use slot had to be set taking account of the delay time period for up to 10 km. Thus, the line accommodation efficiency deteriorates.
An object of the present invention is to provide a PDS structured optical subscriber system that allows a optical subscriber unit to newly subscribe to the system with an entrance use slot whose length is almost a half of a conventional system.
According to a first aspect of the present invention, there is provided a Passive Double Star (PDS) structured optical subscriber system, which comprises: at least one center unit; a plurality of optical subscriber units; a downbound transmission path for transmitting a downbound optical signal from the center unit to the plurality of optical subscriber units; an upbound transmission path for transmitting upbound optical signals from the plurality of optical subscriber units to the center unit; a downbound star coupler for branching the downbound optical signal to the plurality of optical subscriber units; and an upbound star coupler for multiplexing the upbound optical signals from the plurality of optical subscriber units, wherein the center unit comprises: an optical burst signal receiving portion for receiving an upbound optical burst signal from the upbound star coupler; an optical power/delay time period converting portion for inputting the upbound optical burst signal from the optical burst signal receiving portion and converting a power of the upbound optical burst signal into a first delay time period; a response frame detecting portion for inputting the first delay time period from the optical power/delay time period converting portion and detecting a response frame so as to generate a frame detection signal; a delay coarsely-adjusting portion for inputting the frame detection signal from the response frame detecting portion and the upbound optical burst signal from the optical burst signal receiving portion and coarsely-adjusting a second delay time period on the basis of the frame detection signal and the upbound optical burst signal; a delay finely-adjusting portion for inputting the second delay time period from the delay coarsely-adjusting portion and the upbound optical burst signal from the optical burst signal receiving portion and finely-adjusting the second delay time period; a delay time period comparing portion for inputting the first delay time period from the optical power/delay time period converting portion and the second delay time period from the delay finely-adjusting portion and comparing the first and second delay time periods so as to generate a third delay time period which is set to be equal to the second delay time period in case where a difference between the first and second delay time periods is within tolerable range and so as not to generate the third delay time period in the other case; a timing generating portion for inputting the third delay time period signal from the delay time period comparing portion and generating a response timing for each of the plurality of subscriber units; and an optical burst signal transmitting portion for transmitting a first downbound optical burst signal every two frames, and transmitting a second downbound burst signal containing an information on the response timing to each of the plurality of optical subscriber units when inputting the response timing from the timing generating portion, to each of the plurality of optical subscriber units through the downbound star coupler, and wherein each of the plurality of optical subscriber units comprises: an optical burst signal receiving portion for receiving the first and second downbound optical burst signals from the center unit through the downbound star coupler; a received optical power calculating portion for inputting the first downbound optical burst signal from the burst signal receiving portion and estimating an installation distance corresponding to an optical power of the first downbound optical burst signal; a frame determining portion for inputting the estimated installation distance from the received optical power calculating portion and determining a frame in which inserted is the upbound optical burst signal to be transmitted to the center unit as a response to the first downbound optical burst signal; a timing generating portion for inputting the determined frame from the frame determining portion and generating a transmission timing when the upbound optical burst signal is transmitted from the optical subscriber unit; and an optical burst signal transmitting portion for inputting the transmission timing from the timing generating portion and outputting the upbound optical burst signal to the center unit through the upbound star coupler.
According to a second aspect of the present invention, there is provided a PDS structured optical subscriber system, which comprises: at least one center unit; a plurality of optical subscriber units; a downbound transmission path for transmitting a downbound signal from the center unit to the plurality of optical subscriber units; an upbound transmission path for transmitting upbound signals from the plurality of optical subscriber units to the center unit; a downbound star coupler for branching the downbound signal to the plurality of optical subscriber units; and an upbound star coupler for multiplexing the upbound signals from the plurality of optical subscriber units, wherein the time period of an entrance use slot is shorter than a turnaround time period of a signal transmitted through the maximum length of the upbound transmission path and the maximum length of the downbound transmission path.
According to a third aspect of the present invention, there is provided an optical subscriber unit used for a PDS structured optical subscribe system, which comprises: detecting means for detecting whether an installation distance of the optical subscriber unit is in a short distance range or a long distance range on the basis of a received optical power, when the optical subscriber unit receives an entrance command in a downbound transmission path from a center unit; and transmitting means for transmitting a response burst signal to an entrance use slot of the current frame in a upbound transmission path in case where the installation distance is in the short distance range and for transmitting response burst signal in an entrance use slot of the next frame in the upbound transmission path after a predetermined time period in case where the distance is in the long distance range.
According to a fourth aspect of the present invention, there is provided a center unit used for a PDS structured optical subscriber system, which comprises: first installation distance detecting means for detecting an installation distance of an optical subscriber unit by measuring a delay time period of a response burst signal transmitted from the optical subscriber unit against a subscriber command in a current entrance use slot throughout plural entrance use slots.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the best mode of embodiment thereof, as illustrated in the accompanying drawings.